Versatile coupler for internal reflection spectrometry

ABSTRACT

A versatile coupler for optically coupling a beam of radiation into or out of an internal reflection element for internal reflection spectroscopy is described. The coupler comprises a generally right-triangular member which is optically transparent and two of whose sides are bevelled but in opposite directions. In use, the vertically oriented beam enters at one side of the triangular coupler, internally reflects off its major surfaces and the hypotenuse and exits from the coupler at the opposite side from whence it enters the internal reflection element which is in optical contact with said opposite side.

u/uiklil v|\l.l\ ll rATENTS 3/1939 Great Britain OTHER REFERENCESHirschfeld, Procedures For Attenuated Total Reflection Study ofExtremely Small Samples," April 1967. Applied Optics, Vol. 6, No. 4, pp.715- 718 Primary E.raminer David Schonberg Assistant Examlner Michael.1. Tokar ABSTRACT: A versatile coupler for optically coupling a beam ofradiation into or out of an internal reflection element for internalreflection spectroscopy is described. The coupler comprises a generallyright-triangular member which is optically transparent and two of whosesides are bevelled but in opposite directions. In use, the verticallyoriented beam enters at one side of the triangular coupler, internallyreflects off its major surfaces and the hypotenuse and exits from thecoupler at the opposite side from whence it enters the internalreflection element which is in optical contact with said opposite side.

PATENTEUIJEB H971 3,625,587

sum 2 OF 2 Fig. 9

Fig. ID

Fig. ll

Fig. I5 42 f Fig. l2

INVIEN'I'UR.

N. J. HARRICK VERSATILE COUPLER FOR INTERNAL REFLECTION SPECTROMETRYThis invention relates to intemsl reflection spectroscopy, and inparticular to an optical coupling member for coupling radiation into andout of an internal reflection element.

lntemal reflection spectroscopy is now a well established technique forobtaining optical spectra of a sample material. The sample material,which may be solid, powder, liquid, or thin film, is brought intocontact with the totally reflecting surface of the internal reflectionelement, through which a beam of radiation is passed so as to totallyinternally reflect off of the surface containing the sample material.The evanescent wave from the radiation can interact with the sample andthe extent of interaction is recorded by measuring the reflected orexiting beam intensity versus wavelength.

The internal reflection element is usually in the form of a thin plate,with the beam of radiation multiply reflecting from the plates majorsurfaces of which at least one contains the sample material. Details ofthis spectroscopy technique including a complete bibliography,discussion of principles, instrumentation, and applications can be foundin my book entitled Internal Reflection Spectroscopy," InterscienceDiv., J. Wiley & Sons, N.Y., 1967, whose contents are herebyincorporated by reference.

For many applications it is preferred to arrange the internal reflectionelement horizontally with the sample material located on its top majorsurface. This arrangement causes difficulties. A major problem is thatthe beam receiving aperture of the element is thus orientedhorizontally, whereas the slits of the spectrometer are conventionallyoriented vertically. In order to introduce the beam into the element,the beam orientation has to be rotated 90. One way of achieving this isto use a prism tilted at 45, plus a mirror as shown on page 104 of myaforementioned book. A second similarly oriented prism plus mirror wouldthen have to be used to extract the beam. This is a cumbersome solution.a

On page 108 of my aforementioned book, 1 illustrate a verticallyoriented double-pass multiple internal reflection plate having arooftop-bevelled entrance and exit surface. This element will receive avertically oriented beam. A difl'rculty encountered with this element isthat, in use, the constant contacting of the surfaces with the samplematerial causes damage to them so that its total internal reflectionproperties suffer. Such plates have to be repolished to perfonnproperly. The repolishing of a plate of such complex geometry isdifficult and time-consuming.

One object of my invention is a novel coupling element for introducing avertically oriented radiation beam into an internal reflection elementhaving a horizontally oriented aperture.

A further object of the invention is a double-pass multiple reflectioninternal reflection plate capable of receiving a vertically orientedbeam but of simpler geometry than that known heretofore for easierreprocessing.

Still another object of my invention is an optical coupling member forinjecting and extracting a beam of radiation into and out of an internalreflection plate and capable of being used with many different kinds ofinternal reflection plates in many different geometries.

These and other objects of the invention as will appear hereinafter areachieved with a coupling element having a flat generally triangularshape of which the sides form a substantially right angle. The sides areconstructed, as by bevelling for instance, to enable the incident beamto enter the member and impinge on a major surface at an anglepermitting reflection therefrom onto the opposite major surface so as tocause the beam to propagate through the member by multiple reflectionsand such that it also reflects off of the hypotenuse sur face, to exitfrom the member in a direction to enter the internal reflection elementat the appropriate angle for internal reflection spectroscopy. In oneembodiment, the coupling member is a solid prism. In another embodiment,the coupling member is a hollow body.

The invention will now be described in greater detail with reference tothe accompanying drawings, wherein: FIG. I is a plan view of one form ofsolid optical coupling member of the invention; FIGS. 2 and 3 are sideand front elevational views respectively of the coupling member shown inFIG. 1; FIGS. 4-9 show the coupling member of FIG. I coupled to variousforms of internal reflection elements in different arrangements, withFIG. 6 being a side view of the perspective arrangement shown in FIG. 5;FIG. 10 is a plan view of a modified coupling member of the invention,and FIGS. 11 and 12 show how this modification may be used with aninternal reflection element; FIG. 13 is a further modification of mynovel coupling member; FIGS. 14 and 15 are still other modifications ofmy coupling member of the invention but in hollow form, with FIG. 14being a perspective view and FIG. 15 a plan view.

FIGS. 1, 2 and 3 illustrate one embodiment of the coupling member of myinvention. It comprises a solid body I of optically substantiallytransparent material in the form of a righttriangular thin flat plateall of whose surfaces are planar and optically polished. It comprisesopposite major parallel surfaces 2, 3, a hypotenuse edge surface 4perpendicular to the major surfaces, and side edge surfaces 5, 6 whosecenter lines form a right angle. The side edge surfaces 5, 6 arebevelled in opposite directions to form angles of 45 with the majorsurfaces 2, 3. The entrance for the beam of radiation 8 is the side edgesurface 5, and the exit is the adjacent side edge surface 6. Thespectrometer slits are usually oriented vertically, producing a narrow,rectangular, vertically oriented beam, which is directed nonnally at the45 bevelled entrance surface 5 so as to substantially fill the aperture.The beam thus impinges on the back major surface 3 at a 45 angle. Thematerial is chosen to have an index of refraction such that its criticalangle is below 45. Thus, the beam is totally internally reflected, andcan propagate horizontally (FIG. I) through the member by multiplereflections from the major surfaces 2, 3. The geometry is such that thebeam will also totally internally reflect off of the hypotenuse sidesurface 4, then will propagate vertically through the member by multiplereflections from the major surfaces 2, 3, and finally will impinge onthe exit surface 6 at right angles and pass out ofthe member as a beam9. As will be observed, the exiting beam 9 will still have the form of anarrow rectangle, but now oriented horizontally instead of vertically,and thus rotated relative to the incident beam 8, but also propagatingat 45 relative to the horizontal (FIG. 2).

FIG. 4 illustrates one way of using the coupling member of FIG. I toform a vertical double-pass cell performing a similar function to thatillustrated on page 108 of my aforementioned book, except thatdouble-sampling also is present. In this case, the coupling member 1 isoptically contacted to a multiple reflection internal reflection element10 in the form of a plate of the same thickness as that of the couplingmember I and also having a similarly 45 bevelled entrance surface 11.The entrance surface 11 of the element 10 is contacted to the exitsurface 6 of the coupling member 1. To improve the contact, a drop ofoptically transparent liquid having an index comparable to that of theelements, such as, for example, Nujol, may be employed. The lower end 12of the element 10, instead of being cut at an angle of 90 relative toits major surfaces, on which the sample is provided, is cocked at anangle of approximately I2/n, where n is the index of refraction, asdescribed on page I05 and illustrated on page I07 of my aforementionedbook to separate the exit beam from the entrance beam. This wouldprovide a beam separation of about 24. In operation, the verticallyoriented entrance beam 13 impinges normally on the entrance surface 5 ofthe coupling member, then propagates through it by multiple internalreflections from the major surfaces 2 and 3, in the process totallyinternally reflecting off of the hypotenuse side surface 4, exits via.the surface 6 and passes into the element I0, propagates downwardthrough the internal reflection element 10 similarly to the mannerillustrated in page I08 of my aforementioned book, totally internallyreflects from the cocked end surface 12, propagates upwardly through theelement by multiple internal reflections, reenters the coupling member1, propagates through it by multiple internal reflections in the processreflecting off of the side surface 4, and exits as a beam 14 via theentrance surface 5 but at an angle from the incident beam 13 equal tothe refractive index of the material times the angle of tilt of thecocked end 12. As will be noted, the element can be readily separatedfrom the coupling member 1, and thus any damage by the sample materialto the element's surfaces is easily repaired by polishing only itssurfaces.

FIGS. 5-9 illustrate various ways in which my novel coupling element 1may be employed to couple a vertically oriented beam from a spectrometerinto a horizontally oriented internal reflection element. As will beclear from Chapter VI of my aforementioned book, the beam available fromthe commercial spectrometers is oriented in a vertical direction. Usingsuch a beam in a multiple reflection plate requires that the plate beoriented vertically. This is also illus trated in U.S. Pat. No.3,332,315. It is often much more convenient to orient the platehorizontally, for ease of applying the sample material. This is readilyachieved with my novel coupling member. FIGS. 5 and 6 show the member 1oriented to receive a vertical beam 16 and couple same into ahorizontally arranged double-pass plate 17 having a 45 bevelled entrancesurface 18 matched to the exit surface 6 of the coupling member. Theopposite end 19 is bevelled and cocked as previously described. If thegeometry is such that total internal reflection off of that cocked endis not possible, a reflecting metallized film 20 may be providedthereon. The beam, after multiple reflections through the member 1 andthe element 17, exits as a beam 21 from the member, reorientedvertically and thus can reenter the spectrometer.

FIG. 7 shows a side view of a single-pass multiple reflection plate 25with 45 bevelled entrance 26 and exit 27 end surfaces to which arecontacted a pair of the coupling members 1 of the invention, one forcoupling in the entrance beam 28 and one for extracting the exit beam 29after interaction with a sample on the top major surface of the plate25. FIG. 8 shows the identical geometry but inverted, which will providea more convenient arrangement for supporting the input and outputcoupling members 1 in contact with the plate 25.

FIG. 9 illustrates an arrangement similar to FIG. 7 except that theplate 30 serving as the internal reflection element has approximatelyone-half the thickness of the coupling members 1 and only contacts thelower half of the bevelled surfaces, the remaining space being occupiedby the sample material 31, in this case a liquid sample, held in placeby the upwardly projecting edges of the coupling members I.

FIGS. 10-12 illustrate in a plan view a modified form of coupling memberhaving only one bevelled edge 41 at to its major surfaces, the adjacentedge 42 being orthogonal to the major surfaces similarly to thehypotenuse edge 43. FIG. 11 illustrates in a side view its use with adouble-pass plate 43, whose bottom major surface is contacted by theorthogonal edge 42. An advantage of this arrangement is that thecoupling member 40 can be readily shifted along the plate 43 to changethe number of multiple internal reflections therein. FIG. 12 shows thecoupling member 40 butted up against the end sur face of the plate 43,which is another way in which it can be used.

FIG. 13 shows in a plan view a further modification of the couplingmember 1 which has been extended upward to provide a short rectangularsection, designated 45, designed to support a suitable mounting tab 46.The lower part of the member, designated 44, remains the same as thatillustrated in FIG. 1 with an entrance bevel 47, but the exit bevel 48has been moved to the top edge of the rectangular extension 45.

So far, the coupling member described has been a solid body akin to aprism, designed of a material to transmit the beam to the internalreflection element. Air is also a suitable substantially transparentmaterial. FIG. 14 illustrates a modiflcation using air and mirrors toreorient the beam in the direction desired. It comprises a hollowtriangular member 49 composed of bottom and top planar parallel elements50, 51

having internal mirror or reflecting surfaces forming a right trianglewith a side edge element 52 also having an internal reflecting surfaceand forming the hypotenuse of the right triangle, whose sides 53, 54,which are open, form the entrance and exit apertures for the beam asshown. The sides being open are not bevelled, but the coupling member 49will have to be supported relative to the beam and to the internalreflec tion element such that the beam, designated 55, impinges on themajor reflecting surfaces at an oblique angle so that it propagates bymultiple reflections as shown through the ho!- low member. Such aconstruction is not as efficient as the prism embodiment of FIG. 1because the reflectivity ofa mirror is never quite as high as totalinternal reflection, and thus some beam power is sacrificed. FIG. 15illustrates a relatively simple way of achieving the geometryillustrated in FIG. 14 and with edges corresponding to a bevel contact.It comprises top and bottom rectangular plates 60, 61 with metallizedreflecting inner surfaces which are overlapped as shown on oppositesides of a flat triangular spacer member 62 whose hypotenuse 63 ismetallized to form a reflector. The plates 60, 61 may be cemented to thespacer 62. The right short edge 63 of the bottom plate 61 together withthe right long side edge 64 of the top plate 60 defines a planeextending at a 45 angle to the planes of the plates 60 and 61.Similarly, the upper short edge 65 of the top plate 60 defines with theupper long edge 66 of the bottom plate 61 at 45 plane, These planes correspond to the bevelled edges 5 and 6 of the embodiment of FIG. 1. Thus,when the upper edges are contacted to the bevelled edge ofthe verticalplate 10 of FIG. 4 or the horizon tal plate 17 of FIG. 5, the couplingmember will be properly oriented thereto. An advantage of these hollowembodiments is that no reflection losses occur at the entrance and exitapertures of the member, and more importantly, optical contact to theplate is not required.

In the various embodiments described above, except for that of FIG. 14,the bevel angle of the entrance and exit apertures of the couplingmember has been 45. This is not essential to the invention. The anglecan be varied over a range of approximately critical angle to grazingincidence for most materials. For the solid member, 45 is preferredbecause it provides the largest aperture and the normal incident beamwill impinge on the major surface at an angle of incidence of 45 whichwill exceed the critical angle for most materials, and the exiting beamwill also be normal to the exiting surface. This reduces reflectionlosses. Also, the 45 angle makes the coupling member more universallyapplicable with many different kinds of internal reflection elementshaving similarly angled bevels. If the bevel were chosen at a smallerangle, care would have to be taken to ensure that the angle of incidenceon the major surface exceeds the critical angle. However, it will befurther understood that the incident beam does not have to be normal tothe entrance surface of the solid coupling member but can be varied overa liberal range, though of course some reflection losses will ensue. Itis further noted that with a double-pass arrangement, the sample elementhaving a cocked end, the exiting beam will not be normal to the entrancesurface (now also serving as an exit surface) and the refraction thatthus takes place provides the required separation of input from outputbeams. With the hollow embodiment, an even wider range of angles ispossible since specular reflection rather than total reflection isemployed.

The materials of which the coupling member may be composed are the samematerials of which the intimal reflection element may be composed, andmany examples of suitable materials may be found in Chapter IV, SectionF of my aforementioned book, which need not be repeated here. Further,suitable methods of preparation are also described in great detail inChapter IV, Section E.

The geometry requirements for the coupling members are, again, similarto those for the internal reflection element, and this subject matter istreated in detail in Chapter IV, Section E, Subsection 3 of myaforementioned book. The optical coupling member of my invention is aspecial form of multiple reflection plate with the difl'erences that nosample is provided on it and it only serves to inject and extract thebeam from the active internal reflection element. Thus, the geometry canbe chosen to provide any number of suitable multiple reflections of thebeam between the major surfaces between the beam exits from the memberat the desired angle and passes into the internal reflection element. Asone suitable example, which is not to be regarded as limiting, to couplea beam having a length of 16 mm. and a width of about 2.12 mm. into aninternal reflection element having an aperture of 16 mm. and a thicknessof 1.5 mm., bearing in mind the desirability of keeping the number ofreflections down and the amount of material small to reduce losses, lprovide a CaF, prism as illustrated in FIG. 1 having sides ofapproximately 19.5 mm., a hypotenuse of 27.6 mm., and a thickness of 1.5mm. With entrance and exit bevels at 45, and a normal beam, the angle ofincidence is 45 (0c for CaF,-Air is about 43), and a ray at the centerof the beam undergoes six reflections from the major surfaces before itreflects off of the hypotenuse surface, and six reflections afterwardsbefore exiting normal to the exit surface. Also, while I have shownbevels on the side surfaces in opposite directions, it will beunderstood that the bevelled sides of, for example, the embodiment ofFIG. I can be in the same direction. in certain instances, this may bepreferred, as such an embodiment can serve either as a left or righthanded unit for coupling to either end of the internal reflection plate.

It will be evident that my invention provides aremarkably versatilecoupling member capable of greatly facilitating the use of internalreflection spectroscopy by affording improved arrangements of theinternal reflection element, and simplifying the reconditioning of suchelements. While I have described my invention in connection withspecific embodiments and applications, I wish it to be understood that Ido not intend to be limited thereby as various other modifications willreadily suggest themselves to those skilled in this an without departingfrom the spirit of my invention.

What is claimed is:

l. A coupling member for use with a separate internal reflectionelement, comprising a generally triangular substantially opticallytransparent body having top and bottom planar parallel reflecting majorsurfaces separated by a distance small relative to the size of the majorsurfaces and a side diagonal planar reflecting surface extending betweenand normal to the major surfaces, said side diagonal reflecting surfaceforming an angle of 45 with opposite adjacent edges of the majorsurfaces, one set of edges adjacent one end of the side diagonalreflecting surface forming a substantially nonreflecting area oppositelylocated with respect to said side diagonal reflecting surface forreceiving a radiation beam image oriented in a given plane, the otherset of edges adjacent the opposite end of the side diagonal reflectingsurface forming a substantially nonreflecting area oppositely locatedwith respect to said side diagonal reflecting surface through which theradiation beam can exit after reflection off of the major reflectingsurfaces and the side diagonal reflecting surface, said body beingpositioned so as to receive the radiation beam image into its receivingarea at an oblique angle causing the beam to reflect off both majorsurfaces and the side diagonal reflecting surface and exit through theexit area at an oblique angle relative to the major surfaces and rotatedsubstantially to its incident given plane, wherein the top reflectingsurface edge at the other set of edges extends beyond the bottomreflecting surface edge thereat. whereas the bottom reflecting surfaceedge at the one set of edges extends beyond the top reflecting surfacethereat, the edges of each set define a plane extending at an angle of45 to the major surfaces, and the interior of the member is hollow.

2. The coupling member set forth in claim 1 in combination with aseparable multiple reflection internal reflection plate having anentering surface in contact with the other edge set of the couplingmember.

3. The combination of claim 2 wherein the coupling member is supportedin a vertical plane, and the internal reflection element is supported ina horizontal plane, and the incident beam of radiation is oriented in avertical plane.

4. A coupling member for use with a separate internal reflectionelement, comprising a generally right-triangular shaped thin flat bodyhaving planar, parallel major surfaces and side edges and a reflectinghypotenuse edge extending at an angle of approximately 45 to the sideedges, both side edges being bevelled adapted to receive a beam ofradiation at an angle to the plane of the major surfaces, said bodybeing constituted of substantially optically transparent material suchthat a beam of radiation incident on one of the bevelled edges willpropagate through the body by multiple reflections from the majorsurfaces, the other side edge enabling the beam to exit from the memberafter reflection from the hypotenuse, the exiting beam being rotatedsubstantially 90 with respect to the incident beam the coupling memberbeing adapted to mate with a separable multiple reflection internalreflection plate having a surface in contact with one side edge of thecoupling member and oriented relative thereto such that the beam ofradiation exiting from the coupling member enters the internalreflection plate to impinge upon the surfaces thereof at an angleexceeding the critical angle.

5. A coupling member as set forth in claim 4 wherein the member isoriented relative to the incident beam such that the angle of incidenceof the beam on the major surfaces exceeds the critical angle.

6. A coupling member as set forth in claim 5 wherein both side edges arebevelled flat at an angle of 45 relative to the major surfaces.

7. A coupling member as set forth in claim 1 wherein the reflectingmajor surfaces are metallized.

8. A coupling member as set forth in claim 5 wherein the other side edgeextends at right angles to the major surfaces.

9. The combination set forth in claim 4 wherein the contacting surfacesof the coupling member and internal reflection plate are bevelled flatat the same angle, and the thickness of the coupling member is of thesame order as that of the internal reflection plate.

10. The combination of claim 9 wherein the end of the internalreflection plate remote from the coupling member is cocked at an angleto cause the returning beam to reimpinge on the entrance surface of theplate at an oblique angle.

II i i i i

2. The coupling member set forth in claim 1 in combination with aseparable multiple reflection internal reflection plate having anentering surface in contact with the other edge set of the couplingmember.
 3. The combination of claim 2 wherein the coupling member issupported in a vertical plane, and the internal reflection element issupported in a horizontal plane, and the incident beam of radiation isoriented in a vertical plane.
 4. A coupling member for use with aseparate internal reflection element, comprising a generallyright-triangular shaped thin flat body having planar, parallel majorsurfaces and side edges and a reflecting hypotenuse edge extending at anangle of approximately 45* to the side edges, both side edges beingbevelled adapted to receive a beam of radiation at an angle to the planeof the major surfaces, said body being constituted of substantiallyoptically transparent material sucH that a beam of radiation incident onone of the bevelled edges will propagate through the body by multiplereflections from the major surfaces, the other side edge enabling thebeam to exit from the member after reflection from the hypotenuse, theexiting beam being rotated substantially 90* with respect to theincident beam the coupling member being adapted to mate with a separablemultiple reflection internal reflection plate having a surface incontact with one side edge of the coupling member and oriented relativethereto such that the beam of radiation exiting from the coupling memberenters the internal reflection plate to impinge upon the surfacesthereof at an angle exceeding the critical angle.
 5. A coupling memberas set forth in claim 4 wherein the member is oriented relative to theincident beam such that the angle of incidence of the beam on the majorsurfaces exceeds the critical angle.
 6. A coupling member as set forthin claim 5 wherein both side edges are bevelled flat at an angle of 45*relative to the major surfaces.
 7. A coupling member as set forth inclaim 1 wherein the reflecting major surfaces are metallized.
 8. Acoupling member as set forth in claim 5 wherein the other side edgeextends at right angles to the major surfaces.
 9. The combination setforth in claim 4 wherein the contacting surfaces of the coupling memberand internal reflection plate are bevelled flat at the same angle, andthe thickness of the coupling member is of the same order as that of theinternal reflection plate.
 10. The combination of claim 9 wherein theend of the internal reflection plate remote from the coupling member iscocked at an angle to cause the returning beam to reimpinge on theentrance surface of the plate at an oblique angle.